Dynaco SCA-35 Restore

Dave451

AK Subscriber
Subscriber
Hey, folks. Latest project is an SCA-35 that I bought that is "gonna take a lot of love," as the song says. I have read so many good things about this little amp and have found Dave Gillespie's work on it so interesting, that I am going for a total restore using all three of Dave's boards (the pre-amp, power supply upgrade with his Enhanced Fixed Bias circuit, and both output amplifier boards with Dave's 7199-to-6GH8/6U8 conversion). Having done work on Fisher and Scott receivers and amplifiers, I decided it was time to look at Dynaco and these units are very, very well documented here on AK and in various other places on the web, including various vendors with all sorts of parts and mods in addition to Dave's very good offerings.

Below are a few "before shots"--this one looks like it was stored in a barn for 15 or 20 years. The chassis and bottom plate are heavily rusted around the transformers, things are very dirty with corrosion in several places and the workmanship on the assembly is suspect, particularly the soldering. The output circuit boards have seen a lot of heat and all the switches and controls need thorough cleaning or replacement. The plug was cut off the power cord and the fuse and holder cap were missing. The back of the chassis was bent.

The front panel and the knobs are not bad, but there are some dings in the upper edge that will have to be addressed.

The tube complement was quite good. RCA 12AX7's and 7199's all tested quite strong for mutual conductance. For 6BQ5's, two RCA that look like GE's (with the dots), one Dynaco label, and one GE label. All but the Dynaco tested good and fairly well matched at tube checker conditions, but we'll see how they look under power.

Obviously, I didn't even try to power this one up and instead decided to do pretty much a complete tear down and restore. We'll see how that goes in the next few weeks.
 

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yeah, the moisture is not a great sign. That one might be worth pulling the boards and putting in the oven for a long, low bake to dry them out.
 
Truer words were never spoke, Mike. The OPT's all check good for DC resistance. Continuity was also good for the power transformer windings. Today, I hooked the PT to my Variac and dim bulb tester and gingerly turned the knob. Checked fine, with around 315/320VAC between HV secondary and center tap, about 630/640 VAC across the secondary, and right around 6.5 VAC on each of the filament windings. No smoke or heat (so far!).
Dave
 
I'm going to set the original boards aside and do a complete make over with Dave's boards. Already got the boards and the parts in and am cleaning things up before getting started on the boards. As per above, transformers were the first critical check. I lightly brushed the laminates with a "Scotch Brite" pad and sanded/wire brushed the rust off the bells. Going to paint them satin black tomorrow and get started on the circuit boards.
Dave
 
Well, I engaged my brain and remembered how to AC test output transformers. I used my Variac and tested each transformer at 3VAC RMS 60 Hz into the primary and again using 1 kHz at 1.0 VAC RMS. Calculating the turns and impedance ration, and finally the primary impedance I got 7500 and 7900 ohms for the respective transformers against the spec of 8000 ohms. So, I think we're in business and I went ahead painting the OPTs and the power transformer.

I also went at it with the chassis and the back panel with Naval Jelly (multiple applications), and then light sanding and finishing with fine steel wool (fibers of which I scrupulously removed when done). Hit the bottom plate as well. Things look much better now (pictures below), although the rust was so deep on the chassis that it left a fair amount of pitting. Back plate cleaned up very well, so I cleaned up and remounted the hardware on it (straightened the bend out easily by hand after removing it from the main chassis). Because of the pitting, I thought of painting the top of the chassis, but decided to leave it as is.

Getting the rust and dirt off the back of the back panel was a challenge without removing the original lettering. In some places where the rust was heavy, I did lose some lettering. I could have just cleaned it down to metal and painted it as well to match the chassis, then put some labels on it, but I thought it was better to keep the original look with some "patina" to show the amp's 'veteran status.' The two hum pots were pretty much a total loss, so I installed new.

Going to start building the boards now and then it will be hook up time, which will be fun with mostly new wiring. Fortunately, I have the original assembly manual, Dave's excellent support documentation with the new boards, and plenty of pictures of the original to guide me.

Dave
 

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When installing the new boards, observe and maintain the same lead dress as per the original. I would use the same multi-strand cables.
 
Thanks, guys. Dandy, yep I'm going to keep the multi-strands and just clean them up a little. I will use the same lead dress, including twisting of the pairs etc., used in the original. Got the pre-amp and power supply upgrade/EFB boards populated last night. Still have to do the two output boards. Transformers are now painted and re-assembly will start soon. My plan is to keep the front end components (wired in the front panel) to preserve as much of the original signal path as possible, unless I find defective/out-of-spec couplers or resistors (I'll test them).
 
Finished up the circuit boards. There are 3: PC-11 pre-amp board, EFB/Power supply board, and the two final amp boards PC-10A. Went together nicely thanks to Dave's excellent step-by-step instructions and a little discipline on pre-assembling and organizing the parts, etc. I'll give them a third check before installing. Also shown are the spiffed up transformers, ready to reinstall. Time to start re-wiring and re-assembling!
Dave
 

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More progress. Mounted the transformers on the chassis (used grommets for feeding the wires through the chassis--none on the original unit) and the two PC-10A boards and the PC-11 pre-amp board. Also re-wired the back chassis and corrected a couple of things (involving electrical tape insulation) on the front panel. Shaping up!
Dave
 

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Hey, one question for you guys: the assembly manual keeps referring to a "pictorial diagram" that assigns numbers to the various input and output terminals, switches, potentiometer lugs, etc. Try as I might, I can't find this reproduced in any of the manuals on line, anywhere or in AK. Anyone know where there may be a scanned copy available? Thanks.
Dave
 
I am enjoying this thread very much. Mostly because you are doing something that I don't have the skills to do (I envy you) I have a question for you, are you going to keep the original tone control circuits? Or are you going to eliminate them? I have heard that removing them increases the sound quality by quite a bit....
 
removing tonecontols ( which btw does not enhances accuracy) will change "Dynaco SCA-35 Restore"
to "Dynaco SCA-35 Modification"
 
Audiosoul,
Sorry--been away a couple of days and just getting back on the bench with the SCA-35. No, I don't intend to modify the tone controls or eliminate them. The mods are limited to the power supply upgrade and installation of Dave Gillespie's Enhanced Fixed Bias circuit (same upgrade PCB), installing the modified PC-10A boards (again Dave G's) to switch from the harder-and-harder-to-find 7199 tube to the plentiful 6U8/6GH8 tube, and a new (but same configuration as stock) PC-11 pre-amp board.

Will send more pictures soon. Things have been "interesting" since I'm basically having to follow the installation guides for the new boards and the original installation guide (paper everywhere on the bench!), since I had to disassemble the whole thing to clean it up. So far, there have been a few "do, then un-do's" as the guides conflict depending on which of the other boards are installed and what mods are done to the original circuits. Going to have to triple-check it before applying power!
Dave
 
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